A primary-side phase-shift type DC-DC converter including a full-bridge inverter combined with a rectifier circuit is configured as follows:
That is, the full-bridge inverter comprises a circuit constituted of a reference phase leg and a control phase leg connected in parallel with each other, in which the reference phase leg includes an upper arm including switching element S1, a snubber capacitor C1 connected in parallel to the switching element S1, and a freewheel diode D1 connected in antiparallel to the switching element S1, and a lower arm including a switching element S2, a snubber capacitor C2 connected in parallel to the switching element S2, and a freewheel diode D2 connected in antiparallel to the switching element S2, with the upper arm and the lower arm connected in series with each other; and the control phase leg includes an upper arm including a switching element S3, a snubber capacitor C3 connected in parallel to the switching element S3, and a freewheel diode D3 connected in antiparallel to the switching element S3, and a lower arm including a switching element S4, a snubber capacitor C4 connected in parallel to the switching element S4, and a freewheel diode D4 connected in antiparallel to the switching element S4, with the upper arm and the lower arm connected in series with each other.
The full-bridge inverter converts a DC voltage supplied to its input terminals into a high-frequency AC voltage, and outputs and supplies the AC voltage to the primary side of a transformer connected to the output terminals thereof. To the secondary side of the transformer is connected the rectifier circuit, which rectifies the high-frequency AC voltage output from the transformer. An output smoothing filter removes high-frequency components from the rectifier output, and the resultant DC voltage is supplied to a load.
Power control in the full-bridge inverter configured as described above is performed basically by phase-shift control or PWM control. When the phase-shift control is used, an overlap angle of the control phase with respect to the reference phase is controlled, thereby controlling transmission power. However, a circulation current that circulates between the primary side switching elements and the transformer arises in a power non-transmission period. Although the circulation current produces an effect of making the primary side switching elements perform soft switching operation, the current does not contribute to the power transmission but generates conduction loss in the transformer and the switching elements in the primary side thereof.
On the other hand, when the PWM control is used, it is a control system that turns on the primary-side switching elements only in a power transmission period and does not cause any circulation current. However, the primary-side semiconductors perform hard switching in the PMW control, which will resultantly increase switching loss.
Under such circumstances, a converter has been disclosed in Patent document 1 as the one that can reduce the switching loss in the primary-side phase-shift type DC-DC converter. In the converter shown in this Patent document 1, instead of a smoothing reactor, a fly-wheel diode and a tap-fitted inductor are added to the output stage of the rectifier circuit, a capacitor voltage-dividing circuit is connected between the input terminals of the inverter, and also a resonance reactor is connected between the voltage-dividing point of the capacitor voltage-dividing circuit and an output terminal of the control-phase side of the inverter. Furthermore, snubber capacitors of the control phase leg are discharged or charged through the resonance reactor, whereby the circulation current flowing through the transformer is decreased and switching loss in the switching elements of the control phase can be reduced.